1. Field of the Invention
The present invention relates to monolithic piezoelectric elements, such as monolithic piezoelectric actuators.
2. Description of Related Art
Monolithic piezoelectric elements are known as a type of piezoelectric element that converts electric energy into mechanical energy using an electrostrictive effect of solid material. The monolithic piezoelectric element is formed by alternately stacking piezoelectric ceramic layers and internal conductive layers.
The monolithic piezoelectric element has a similar structure as monolithic ceramic capacitors, and the internal conductive layers are alternately connected to one and the other external electrode. When a voltage is applied to the two external electrodes, an electric field is generated between each of the two adjacent internal conductors, and thus, distortion occurs in the ceramic layers.
However, the electric field is generated to cause distortion only in a region (piezoelectrically active region) where the internal conductive layers overlap when viewed in the stacking direction, and the distortion does not occur in the other region (piezoelectrically inactive region), where the internal conductors do not overlap and hence no electric field is generated.
Consequently, when a large distortion is produced by applying a voltage, a high stress is generated at the boundary between the piezoelectrically active region and the piezoelectrically inactive region. The stress may mechanically break the stack. For example, a large crack may occur between the internal conductive layer and the ceramic layer.
In order to prevent the stack from being broken by the internal stress, Japanese Examined Patent Application Publication No. 6-5794 has disclosed that grooves extending in a direction parallel to the internal conductive layers are formed in a side surface parallel to the stacking direction of the stack. Hence, the invention described in Japanese Examined Patent Application Publication No. 6-5794 alleviates the concentration of the stress by removing part of the piezoelectrically inactive region.
Although Japanese Examined Patent Application Publication No. 6-5794 does not disclose the process for forming the grooves in detail, they may be formed by, for example, using ceramic green sheets on which a vanishing material such as carbon paste has been printed. The vanishing material is eliminated during firing, thus forming the grooves. The grooves may be cut in the stack with a wire saw after firing.
However, these methods have disadvantages.
In the method of eliminating the vanishing material to form the grooves, the piezoelectric ceramic layers are fused and bonded together after disappearance of the vanishing material because the vanishing material disappears at a temperature lower than the firing temperature of the piezoelectric ceramic layers. Consequently, the grooves cannot be formed in a desired shape.
The method using a wire saw is inferior in processing precision, and it cannot be applied to the process using thin ceramic layers. In addition, the processing cost is high, and accordingly the manufacturing cost is increased.